IEEE TRANSACTIONS ON MAGNETICS, VOL 47, NO 10, OCTOBER 2011 3913 Properties of Ru-Doped Ca-Pr Manganate Thin Films Fabricated by PLD Technique Quoc Thanh Phung1;2 , Seong-Cho Yu1 , Duc Tho Nguyen2 , and Nam Nhat Hoang2 BK21 Physics Program and Department of Physics, Chungbuk National University, Cheongju 361-763, Korea Department of Technical Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University Hanoi, Hanoi, Vietnam The Ca0 85 Pr0 15 Mn1 y Ruy O3 (y = 0, 0.04, 0.08, 0.12, 0.16, and 0.20) manganate thin films were prepared by pulsed laser deposition (PLD) technique The X-ray diffraction (XRD) analysis revealed that the samples were single-phased with orthorhombic structure The scanning electron microscopy (SEM) images indicated that the samples were composed of homogeneous grains The Hall-effect measurements showed that the carrier density and Hall mobility of films increased with increasing Ru-doped content The magnetic field dependence of magnetization at various temperatures were measured by using the superconducting quantum interference device (SQUID) and showed that the increase of Ru-doping content induced the large change in broad range of temperature This demonstrates a possible application of these materials in cooling devices as the relative cooling power (RCP) is proportial to (1 )max 1TFWHM (where 1TFWHM is the full width at half-maximum of ) Index Terms—Magnetocaloric effect, manganate, pulsed laser deposition (PLD), thin film I INTRODUCTION II EXPERIMENTAL HE MANGANESE perovskites of form A B MnO (where A La, Nd, Pr, and B Ca, Sr, Ba, etc.) continuously attract the interests of scientists worldwide because of their outstanding electrical and magnetic properties, such as colossal magnetoresistive effect (CMR) [1], and large magnetocaloric (LMCE) effects [2] Recently, among the materials of great focus, the Ru-doped Ca-Pr manganates of Ru O have been extensively studied form Ca Pr Mn as they showed a giant thermoelectric effect (GTE) at high at K) [3] The temperature (i.e., magnetic properties, electrical conduction mechanism, and carrier transport through the grain boundaries of these perovskites were investigated [4], [5] However, a complete understanding of all physical aspects still remains under discussion, which arises in part due to poor control over the quality of samples prepared by conventional ceramic methods [3]–[5] The overall aim of this paper is the fabrication of high-quality Ru O (CPMRO) thin films of composition Ca Pr Mn 0, 0.04, 0.08, 0.12, 0.16, and 0.20) by pulsed (where laser deposition (PLD) technique and as a first step to characterize their structure and magneto-electrical properties The results obtained revealed that the samples prepared by PLD technique possess better quality, with more uniformity in size and boundary of grains The structure of all films showed the orthorhombic symmetry with space group Pnma The transport characteristics of samples were strongly Ru-doping dependent The investigations were also taken on the address of LMCE, and in contrary to the samples prepared by ceramic route, the PLD gave LMCE that might attract attention for potential application in cooling devices T Manuscript received February 20, 2011; accepted May 03, 2011 Date of current version September 23, 2011 Corresponding author: Q T Phung (e-mail: phung.qth@gmail.com) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2011.2154356 The thin films CPMRO with Ru-doping content from up to 0.20 were prepared by PLD using the CPMRO targets of the same compositions The CPMRO targets were fabricated from the pure oxides using a solid-state reaction technique with repeated grinding and sintering at 1180 C for 24 h in open air The CPMRO thin films were then deposited onto a SiO (001) substrate During the deposition, all substrates were heated and kept at 700 C in an oxygen atmosphere of 450 mTorr The process condition was controlled so that the thickness of the samples was kept at the desired value of 30 nm The as-deposited samples were then annealed at 600 C for h in open air The structure and microstructure of the films were characterized by X-ray diffraction (XRD) technique using the Brucker D5005 diffractometer and by the scanning electron microscopy (SEM) using Jeol’s JSM-5400LV microscope The electrical properties of the samples were investigated using the Hall-effect method (Lake Shore 7500/9500) To investigate the LMCE, we measured the field dependence of magnetization at various temperatures by using a superconducting quantum interference device (SQUID), where temperature incremental step and applied field step were 5.0 K and 10 kOe, respectively III RESULTS AND DISCUSSION In Fig 1, we showed the XRD patterns of the as-deposited thin films We can clearly observe that all samples are of single phase and possess the same symmetry The calculation showed that the appropriate space group was Pnma As the maxima are moved left in comparison with that of the standard bulk samples, the lattice structures of as-deposited films were strained [6], [7] This strained state is analogous to the strain-glass state in bulk samples, but is obviously strong by influence of the effect of quenched disorder This strained state disappeared when the films were annealed in air (or in oxygen) In this case, their XDR patterns were quite similar to those of the bulks Table I shows the lattice constants of annealed samples obtained by using the Rietveld refinement method [8] The increase in lattice parameters and volume was observed with 0018-9464/$26.00 © 2011 IEEE 3914 IEEE TRANSACTIONS ON MAGNETICS, VOL 47, NO 10, OCTOBER 2011 Fig XRD patterns of the CPMRO as-deposited films TABLE I LATTICE PARAMETERS AND SOME IMPORTANT MEAN BOND LENGTHS AND BOND ANGLES FOR CPMRO ANNEALED THIN FILMS increasing Ru-doped content Since the radii of Ru (0.565 ) and Mn (0.531 ) cations are similar, the induced changes in lattice parameters were relatively weak, which indicates that the Ru substitution was taken possibly in Mn sites in base matrix The calculation also showed the increase in bond angle (Mn-O-Mn) and bond distances (Ca-O, Mn-O) This led to the reduction in double exchange interaction (DE) since the total overlap integral (which is proportional to the square of cosine of angle per bond distance) reduced The influences of Ru-doping content and of applied magnetic field on Hall mobility and carrier density of the thin films were also investigated The results are shown in Fig 2(a) We observed that the Hall mobility and carrier density of samples remained almost constant when the applied magnetic fields varied The independence of the both on magnetic field was observed even at high fields such as from 1.2 to 1.3 T [Fig 2(a)], therefore the intrinsic elastic energy of strain state of the thin films was uneliminated by external magnetic field [6] This independence of carrier mobility and density on magnetic field argues for the fact that the conduction electrons are not probably involved in the magnetic exchange interactions This observation agrees in principle with the statement about the percolative conduction mechanism in ruthenates and Ru-doped manganates [5] In Fig 2(b), we demonstrate the dependence of Hall mobility and carrier density of thin films on Ru-doped content As seen, these quantities developed almost linearly with Ru-doped content It is worthwhile noting that the values of carrier density developed about 10 times larger (from 10 /cm to 10 /cm ), where the Fig (a) Hall mobility and the carrier density versus magnetic field and (b) Ru concentration for the films The inset in (b) depicts the development of resistivity according to the content of substitution ruthenium content increased from 0.00 to 0.20 In other words, the Ru substitution in B-position heavily influenced the transport properties of the undoped sample Ca Pr MnO In a similar fashion, the Hall mobility of samples also increased approximately ten times when the Ru content increased Since the mobility and carrier density of the samples increased with Ru-doped content, their electrical resistivities decreased [inset, Fig 2(b)] The substitution of Ru cations in the lattice weakly distorted the structure of material as the and Mn ionic radii are quite similar, but the subRu stitution strongly affected the Mn /Mn ratio because of higher valence of Ru in comparison to of Mn Furthermore, when Ru was substituted for Mn, one should consider the FM interactions and consequently the metallicity of samples From the two effects, the double exchange (DE) between Mn and PHUNG et al.: PROPERTIES OF Ru-DOPED Ca-Pr MANGANATE THIN FILMS FABRICATED BY PLD TECHNIQUE 3915 TABLE II RELATIVE COOLING EFFICIENCY (RCP) FOR SEVERAL PEROVSKITE SYSTEMS Fig Temperature dependences of maximum magnetic-entropy changes for Ru-doped samples under magnetic field of T The inset shows the FC-ZFC curve for two samples with y = 0.08 and 0.16 Mn and the super-exchange (SE) between Mn and Ru have to be considered [9], [10] From the inset of Fig 2(b), we can observe that the electrical resistivity of samples was visibly reduced when the Ru content increased The existence of magnetic ordering in general and the competitive coexistence of AFM and FM states in particular suggest that the magnetocaloric behavior may exist in these samples; some enhancements may even occur Indeed, the Ru-doped manganates prepared by the ceramic route showed some magnetocaloric effects, but the relative cooling efficiency was much smaller in comparison with that of the other doped perovskites Pb MnO [12] To investigate the measure of such as La LMCE in the samples prepared by PLD, we recorded the development of magnetization according to temperature and apand evaluated the magnetic entropy change plied field from obtained isotherms according to [13] (1) where is the full width at half-maximum of [11], when Ru content increased therefore the spreading of sufficiently enhanced the cooling power Although the maxima here are a bit smaller than that of La Pb MnO [12] of Cu O [14], the estimated RCPs are and La Sr Mn comparable to that of those compounds (even exceeded for the ) The summary of several RCPs is given in case of Table II The surprising expression of LMCE obtained in the films prepared by PLD is the primary result of this work Such LMCE was not observed in the films fabricated by traditional ceramic route The enhancement of LMCE in Ru-doped manganates may originate in the competitive coexistence of AFM and FM ordering states induced by doping (observe the FC and ZFC curves shown in the inset of Fig 3) The improvement value will be the next step of our of the maximum research IV CONCLUSION We have successfully fabricated the high-quality thin films of Ru O ( 0.00, 0.04, 0.08, composition Ca Pr Mn 0.12, 0.16, 0.20) using the PLD technique The structural and electrical properties of these films have been analyzed and discussed The as-deposited thin films were strained, therefore their structures were deformed but remained orthorhombic, similar to that of the bulk samples after annealing The conductivity of samples showed the increase with increasing Ru-doping content The obtained results indicated that the substitution of Mn by Ru belongs to the electron-doped type The magnetic-entropy changes span over a large range of temperature, and the prepared films can be considered as the potential LMCE candidates for magnetic refrigerators ACKNOWLEDGMENT and represent the entropy in the abHere, sence and in the presence of applied field was evaluated as the function of temperature 0.08, Fig shows the obtained results for Ru contents 0.12, and 0.16 As seen, the maxima occurred at around 0.47 J/Kg K under the field of T One set of maxima (left part) showed the reduction to 0.4 and 0.3 J/kg K, but the other set (right part) remained unchanged Obviously, the increase in a in Ru-doped content induced the large variation of broad range of temperature ( 150 K) This means the enhancement in cooling efficiency and demonstrates that these thin films may be used for cooling devices As defined, the relative , cooling power (RCP) is proportional to This work was supported by the Brain Korea 21 project and the Korea Foundation for Advanced Studies’ International Scholar Exchange Fellowship for the academic year of 2008–2009 The authors would like also to thank the financial support from the National Foundation for Scientific and Technology Development of Vietnam (NAFOSTED), research project “Magnetism in novel perovskite composites” (2010) REFERENCES [1] Y Tokura and Y Tomioka, “Colossal magnetoresistive manganites,” J Magn Magn Mater., vol 200, pp 1–23, 1999 [2] M H Phan and S C Yu, “Review of the magnetocaloric effect in manganite materials,” J Magn Magn Mater., vol 308, pp 325–340, 2007 3916 IEEE TRANSACTIONS ON MAGNETICS, VOL 47, NO 10, OCTOBER 2011 [3] B T Cong, T Tsuji, P X Thao, P Q Thanh, and Y Yamamura, “High Pr MnO ,” J Phys., temperature thermoelectric properties of Ca Cond Matter., vol 352, pp 18–23, 2004 [4] P Q Thanh, B T Cong, C T A Xuan, and N H Luong, “Melting of the charge ordering state by Ruthenium doping in Ru O (y = 0, 0.03, 0.05, 0.07) Perovskite,” J Ca Pr Mn Magn Magn Mater., vol 310, pp e720–e722, 2007 [5] P Q Thanh, H N Nhat, and H D Chinh, “Grain boundary resistivity of the percolative conduction regime in doped ruthenates,” J Magn Magn Mater., vol 310, pp e681–e683, 2007 [6] T Dhakal, J Tosado, and A Biswas, “Effect of strain and electric field on the electronic soft matter in manganite thin films,” Cond-mat./ 0607502v2 [Cond-mat strr-el], 2006 [7] H B Moon, C H Kim, S S Min, J S Ahn, J H Cho, Y K Kim, and R Hyun, “Crystal structures on growth of Pr Ca MnO thin films analyzed by transmission electron microscope,” J Kor Phys Soc., vol 51, p S147, 2007 [8] A Jouanneaux, “WinMProf: A visual Rietveld software,” CPD Newslett., vol 21, p 13, 1999 [9] A Maignan, C Martin, M Hervieu, and B Raveau, “Ferromagnetism and metalicity in the CaMn Ru O perovskites: A highly inhomogeneous system,” Solid State Commun., vol 117, pp 377–382, 2001 [10] L V Bau, N Van Khiem, D N H Nam, L Van Hong, and N X Phuc, “Coexistence and conversion of phases in Ti-doped manganite observed by magnetization and transport measurements,” J Kor Phys Soc., vol 52, no 5, pp 1439–1442, 2008 [11] A M Tishin and Y I Spichkin, The Magnetocaloric Effect and Its Applications Bristol, U.K.: Institute of Physics, 2003 [12] N Chau, H N Nhat, N H Luong, D L Minh, N D Tho, and N N Chau, “Structure, magnetic, magnetocaloric and magnetoresistance Pb MnO perovskite,” Physica B, vol 327, pp properties of La 270–278, 2003 [13] A M Tishin, “Magnetocaloric effect in the vicinity of magnetic phase transition,” J Magn Magn Mater., vol 184, pp 62–66, 1998 [14] N Chau, P Q Niem, H N Nhat, N H Luong, and N D Tho, “Influence of Cu substitution for Mn on the structure, magnetic, magnetocaloric and magnetoresistance properties of La Sr MnO perovskites,” Physica B, vol 327, pp 214–217, 2003 ... al.: PROPERTIES OF Ru-DOPED Ca-Pr MANGANATE THIN FILMS FABRICATED BY PLD TECHNIQUE 3915 TABLE II RELATIVE COOLING EFFICIENCY (RCP) FOR SEVERAL PEROVSKITE SYSTEMS Fig Temperature dependences of. .. the films fabricated by traditional ceramic route The enhancement of LMCE in Ru-doped manganates may originate in the competitive coexistence of AFM and FM ordering states induced by doping (observe... elastic energy of strain state of the thin films was uneliminated by external magnetic field [6] This independence of carrier mobility and density on magnetic field argues for the fact that the conduction